The present disclosure relates generally to apparatuses, methods, systems, and techniques for air-fuel ratio (AFR) imbalance diagnostics. While not so limited, the disclosure finds particular application in the context of spark-ignited engines, such as those fueled by liquid fuels such as gasoline and ethanol and/or gaseous fuels such as natural gas, including pipeline gas, wellhead gas, producer gas, field gas, nominally treated field gas, well gas, nominally treated well gas, bio-gas, methane, ethane, propane, butane, liquefied natural gas (LNG), compressed natural gas, landfill gas, condensate or coal-bed methane (CBM). Such systems frequently utilize an exhaust aftertreatment catalyst whose operation can deteriorate if a cylinder-to-cylinder AFR imbalance is present in the engine, posing a longstanding problem in the art. For example, such systems often utilize a three-way catalyst whose operation deteriorates significantly if an AFR imbalance is present between cylinders, a condition which may be referred to as an inter-cylinder AFR imbalance. Some diagnostics to detect the presence of AFR imbalance have been proposed; however, there remain shortcomings in the performance, reliability, and robustness of conventional approaches. There remains a substantial need for the unique apparatuses, methods, systems, and techniques disclosed herein.